Anaerobically curable compositions generally are well known. See e.g. R. D. Rich, “Anaerobic Adhesives” in Handbook of Adhesive Technology, 29, 467-79, A. Pizzi and K. L. Mittal, eds., Marcel Dekker, Inc., New York (1994), and references cited therein. Their uses are legion and new applications continue to be developed.
Anaerobic adhesive systems are those which are stable in the presence of oxygen, but which polymerize in the absence of oxygen. Polymerization is initiated by the presence of free radicals, often generated from peroxy compounds. Anaerobic adhesive compositions are well known for their ability to remain in a liquid, unpolymerized state in the presence of oxygen and to cure to a solid state upon the exclusion of oxygen.
Often times anaerobic adhesive systems comprise resin monomers terminated with polymerizable acrylate ester such as methacrylate, ethylacrylate and chloroacrylate esters [e.g., polyethylene glycol dimethacrylate and urethane-acrylates (e.g., U.S. Pat. No. 3,425,988 (Gorman)] derived according to known urethane chemistry. Other ingredients typically present in anaerobically curable adhesive compositions include initiators, such as an organic hydroperoxide for example cumene hydroperoxide, tertiary butyl hydroperoxide and the like, accelerators to increase the rate at which the composition cures, and stabilizers such as quinone or hydroquinone, which are included to help prevent premature polymerization of the adhesive due to decomposition of peroxy compounds.
Desirable cure-inducing compositions to induce and accelerate anaerobic cure may include one or more of saccharin, toluidines, such as N,N-diethyl-p-toluidine (“DE-p-T”) and N,N-dimethyl-o-toluidine (“DM-o-T”), and acetyl phenyl hydrazine (“APH”) with maleic acid. See e.g. U.S. Pat. No. 3,218,305 (Krieble), U.S. Pat. No. 4,180,640 (Melody), U.S. Pat. No. 4,287,330 (Rich) and U.S. Pat. No. 4,321,349 (Rich).
Saccharin and APH are used as standard cure accelerator components in anaerobic adhesive cure systems. Indeed, many of the LOCTITE®-brand anaerobic adhesive products currently available from Henkel Corporation use either saccharin alone or both saccharin and APH.
Anaerobically curable adhesive compositions also commonly include chelators such as ethylenediamine tetraacetic acid (EDTA) which are employed to sequester metal ions.
Preparation of anaerobically curable compositions which includes an anaerobically curable component typically involves a liquid carrier component. The composition is thus typically in a liquid form and may be dispensed, for example by an applicator. For use, the anaerobically curable component is applied by a suitable applicator, to form a layer or coating on the surface. Often anaerobically curable compositions are applied as a bead, for example a continuous bead to form a gasket.
The anaerobically curable material often remains wet even after application until exposed to anaerobic conditions suitable for cure. For example in many cases an anaerobic curable composition contains a liquid monomer.
Even though the anaerobic curable composition may dry off somewhat, for example through evaporation (by being dried, or allowed to dry for a period), the material often remains wet and tacky. This leads to: potential contamination of anything that contacts the articles to which the material has been applied and also unwanted removal of the material which has been applied. The latter concern potentially compromises the integrity of any bond or seal later formed by the anaerobically curable composition because an insufficient amount may remain to form the desired bond or seal.
And of course where the liquid carrier material is itself the liquid monomer, it will remain in its liquid form until such time as it is anaerobically cured. So even though these compositions may be applied on a substrate, awaiting exposure to anaerobic conditions, they will remain wet or at least tacky until cured.
In the past additional components such as thickeners have been added to the material to make it less flowable but because other components are liquid the overall composition remains somewhat flowable and/or tacky.
Tape products have existed for example Loctite® 249 Quicktape. This product consists of a liquid anaerobic threadlocker, sandwiched between two films of non-reactive polyamide/polyurethane film.
Compositions, including those that are suitable for use in threadlocking applications may be applied in a dry to touch form but with later stage anaerobic cure functionality. To achieve this additional components are often used.
In some cases a dry to touch form is achieved using a cure mechanism. For example a first cure mechanism may form the dry to touch form so as to hold the composition in place on an article while a second (anaerobic) cure mechanism is activated later to achieve cure, for example to achieve threadlocking.
For example European Patent No. 0 077 659 (Thompson) describes a pre-applied polymerizable fluid for sealing and locking engineering parts. The composition has two mechanisms for curing and two curing reactions take place. The first mechanism is a UV light cure. An opacifier is dispersed in the fluid so that the fluid becomes substantially opaque to radiation. After the fluid is applied to the component it is exposed to UV radiation whereupon a coating is formed, creating a surface layer which is a dry, tack-free crust. The subcutaneous fluid is unaffected by the radiation and remains in a generally liquid state. When the component is threaded into another the surface layer breaks and the second polymerisation (such as a free radical polymerization) is initiated and the second cure reaction takes place. The second polymerization mechanism acts to lock the threads together. In Thompson, only a skin is formed in the first polymerization and the remainder of the composition remains fluid below the skin. There is a risk therefore that during handling of the coated engineering parts the skin may be disrupted and the fluid composition may leak out.
European Patent No. 0 548 369 (Usami) describes a pre-applied adhesive composition for application to the threaded contact faces of a screw screw. The composition comprises a photo-hardening binder in which a secondary curable composition is dispersed. The secondary curable composition includes microencapsulated reactive monomer/activator/initiator.
International Patent Publication WO2004/024841 A2 (Haller) describes curable compositions for application to a threaded article. The composition comprises a dispersion of components of a first cure mechanism comprising: (a) a (meth)acrylate functional monomer component; (b) a (meth)acrylate functional oligomer component; and (c) a photoinitiator component; and (ii) components of a second cure mechanism comprising: (e) an amine component; and (f) an encapsulated epoxy resin component; together with (iii) a thickener component. The photoinitiator component is suitable upon irradiation of the composition to achieve a first cure through the depth of the composition applied to a threaded article so that a binder matrix is formed with the components of the second cure mechanism dispersed through the matrix.
An English language Abstract for Chinese patent publication CN102558490 seemingly discloses a hot-meltable prepolymer, which is an urethane or polyurethane (meth)acrylate prepolymer with (meth)acryloyl terminal groups. The melting point of the prepolymer is 50-80° C. An anaerobic adhesive is prepared from the hot-meltable prepolymer, monomer containing at least one acrylic ester group or methacryloyl group, promoter, stabilizer and initiator. Liquid monomers are combined with the prepolymer to form a gel.
Notwithstanding the state of the art, it would be desirable to provide alternative anaerobically curable compositions that are suitable for typical end use applications including in threadlocking applications.